A conventional transmission apparatus for differential communication includes a driver bridge circuit having a four output devices. The four output devices are independently connected between each of a pair of transmission lines and a power line or a ground line. Two output devices connected to one transmission line are paired to form a first output device pair, and the other two output devices connected to the other transmission line are paired to form a second output device pair. The first and second output device pairs are alternately turned ON and OFF in accordance with data so that differential transmission of data can be achieved.
In the conventional transmission apparatus, potentials of the transmission lines rise or fall, when common mode noise beyond a power supply-voltage range is superimposed on the transmission lines. In this case, even when one output device pair is turned ON, electric current does not flow through one output device of the one output device pair. As a result, the differential transmission of data cannot be achieved normally.
Therefore, measures to reduce common mode noise are taken in the conventional transmission apparatus.
For example, a common mode choke coil is added to the transmission lines. However, since the common mode choke coil are formed with a magnetic core and windings, the apparatus as a whole is increased in size.
JP 2007-318734A and U.S. 2004/0169526 corresponding to JP 2004-253859A disclose an apparatus for reducing common mode noise without using a common mode choke coil. In the apparatus disclosed in U.S. 2004/0169526, a filter circuit for selectively passing common mode noise is provided between a transmission line and a ground line. In the apparatus disclosed in JP 2004-253859A, a common mode potential of transmission lines is measured, and when the measured common mode potential exceeds a threshold potential, output current supplied from a driver bridge circuit to the transmission lines is corrected by a current correction circuit so that a desired differential output can be obtained regardless of the common mode potential
For example, a filter circuit used in the apparatus disclosed in JP 2007-318734A can be formed with resistors and capacitors. In such a case, a waveform of a differential communication signal to be transmitted may be affected by the filter circuit. To prevent this problem, the filter circuit may be formed with a mutual inductance element having a magnetic core wound with a conductor. However, in such a case, the apparatus as a whole is increased in size.
In the apparatus disclosed in U.S. 2004/0169526, since the current correction circuit can be incorporated in the driver bridge circuit, the apparatus can be simplified in configuration without increasing the size.
However, in the apparatus disclosed in U.S. 2004/0169526, a series circuit of two resistors is provided between a pair of transmission lines, and a potential between the resistors is measured as a common mode potential. Therefore, the resistors need to have high impedance in order not to affect a differential voltage between the transmission lines.
When the resistors have high impedance, a time constant, which depends on resistances of the resistor and capacitances between the transmission lines and the ground line, becomes large. As a result, speed for detecting the common mode potential becomes slow. Therefore, for example, it is impossible to follow or track high speed common mode noise with a rising edge of about 100 MHz.
Further, in the apparatus disclosed in U.S. 2004/0169526, the current correction circuit includes a comparator for detecting the common mode potential. The comparator may degrade current correction responsivity.